Create light rays in Cycles
Using a Gradient texture you can fade the volumetric emission off however you want:



In this case, since it should fade out spherically from the center of  the ring I used a spherical gradient:



The vector input of the gradient node defines the position and size of the resulting gradient.
Since in this case we want the center of the gradient to be in the center of the ring, the Object Coordinates (0,0,0 at the object's origin) are what we want. The multiply node scales the coordinate system, which lets us control the size of the gradient.

In the image above the rays are made up of an extruded text object:



For linear gradients, you might not even need the gradient node. For example, the Generated coordinate system has 0,0,0 at one corner of the object's bounding box and 1,1,1 at the opposite corner. Using this, you could fade an object out along it's length by using one of the axis as the gradient: